This is topic sudden level drop - cp750? in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.
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Posted by Carsten Kurz (Member # 5396) on 05-12-2017, 04:33 PM:
A friend just called me from the middle of a festival. Suddenly they had to push up their CP750 to 8 or 9 to get a decent level on their system. They tried with different content, but the problem remained. They usually play around 4. Anyone ever experienced that with a cp750? They said they can rule out the amps. Their server has no digital audio leveling capability. A dying power-supply? Overheating?
- Carsten
Posted by Stephan Shelley (Member # 8587) on 05-12-2017, 06:43 PM:
Have you checked the power amps? I have recently had issues with the vol pots on the OSC DCA amps. They can cause a vol drop or sound cutout.
Posted by Richard Fowler (Member # 893) on 05-12-2017, 07:08 PM:
I found the same volume issues for a festival where the installer did not allow proper air flow and was running hot
Posted by Mark Gulbrandsen (Member # 72) on 05-12-2017, 07:33 PM:
I could make a good living just traveling the country cleaning QSC amps... If you clean the pots be sure to also clean the heat sinks.
Posted by Marcel Birgelen (Member # 6801) on 05-12-2017, 09:05 PM:
They probably have more than one amp and all of them failing at the same time is highly unusual.
They probably have done some power cycling already? I'm not sure how this could be a power supply related failure, since it would be kind of an odd failure only to affect the pre-amp/analog part and not the digital/controller part of the processor.
I guess they're using digital inputs? In the unlikely case they're not, they might check the input side of things obviously.
Posted by Stephan Shelley (Member # 8587) on 05-12-2017, 09:47 PM:
You loose center channel you loose a lot and if it is a 4 channel amp.... Had one of those have dirty pots.
Posted by Carsten Kurz (Member # 5396) on 05-13-2017, 05:52 AM:
They were in the mid of a festival screening when they called. They didn't bother to pull up the CP to 9 as long as they were there. They WERE bothered what would happen if for some reason 'the old' situation would come back and blast peoples heads off.
After that screening round, they powercycled, and placed a fan-blower on the CP750 and the issue was gone. Usually analog output amps would be sourced by the +/-15 Volts, so I would have suspected something around this. With some basic measurement gear it would have been easy to diagnose the call, but they don't have the gear and the knowledge to do this, especially not in the mid of a festival. As they are a bit far off the rest of the world, I advised them to get a backup power-supply for the 750.
Does anybody know how the output levels of the CP750 are set? Purely digital, or with digitally controlled pots?
- Carsten
Posted by Richard Fowler (Member # 893) on 05-13-2017, 07:31 AM:
The festival situation they had the CP750 stacked on equipment like books on a shelf with zero air flow and hot to the touch. The booth air conditioning was off. The pedestal was mounted backward with rack face to the front wall for non-easy service.
They spent 16 hours to install the equipment versus 4-6 hour average for the site. Did the same...relocate....power cycle...cool down with fan and AC. The "installer" company run great ads of their skills in the trade magazines. ![[Confused]](confused.gif)
Posted by Carsten Kurz (Member # 5396) on 05-13-2017, 08:24 AM:
Whatever creates service calls is good for them...
During the gold-rush of the digital rollout, I saw lot's of installations like that.
- Carsten
Posted by Dave Macaulay (Member # 813) on 05-14-2017, 01:07 PM:
The CP750 has one PCB, no adjustments on it. They do not release a schematic so I don't know how the signal path looks, but it's probably totally digital until the final D-A stage. There would be no reason to go analog and insert a fader stage afterwards.
Posted by Harold Hallikainen (Member # 5405) on 05-14-2017, 02:57 PM:
I don't know about the CP750, but USL processors like the JSD-60 and JSD-100 have an SPI controlled analog pot between the DAC and output line driver. Gain trim is done here to maintain maximum dynamic range. If a trim down is done before the DAC, the signal goes down, but the noise does not.
With this particular issue, did all channels go down in level? Does the CP750 front panel bar graph show the levels down? Does a booth monitor also reveal the levels down?
Good luck!
Harold
Posted by Carsten Kurz (Member # 5396) on 05-15-2017, 04:29 AM:
Hi Harold - I told them to check these indications. However, in the mid of s screening series, their willing to do a proper analysis were limited. They were just glad it worked after rebooting and applying the fan blower...
- Carsten
Posted by System Notices (Member # 2357) on 10-20-2018, 11:06 AM:
It has been 523 days since the last post.
Posted by Steve Guttag (Member # 268) on 10-20-2018, 11:06 AM:
Dolby has issued a field Bulletin on this. They claim it is caused by cooking the unit (not enough ventilation) and the repair is changing the motherboard.
Posted by Ken Lackner (Member # 1002) on 10-23-2018, 04:12 PM:
Yep. Had this happen recently. Norm at Dolby confirmed it's a known issue with the CP750 and I had to send it in for repair. The motherboard was, indeed, replaced. I wasn't told that it was caused by overheating, but that doesn't mean it wasn't. I don't see how, though. There's nothing above the 750 in the rack, and there is a blank space between it and the monitor below.
Posted by Dave Macaulay (Member # 813) on 10-23-2018, 09:29 PM:
We have had a few issues with them doing odd things when overheated. I have been rearranging racks on service visits to ensure the 750 is not right above anything warm, and has at least one RU open above and below. May not be a cure-all but it can't hurt. The cooling is 100% convection with no front to back venting and not a lot of venting holes around the psu, top and bottom and some on the side. Some racks have almost no rear venting so front to back cooled amps heat up the inside, I have removed some rear doors as well.
This was a serious problem on a system with Bryston 4B "toaster" amps in a packed small rack, we added fan cooling.
Posted by Steve Guttag (Member # 268) on 10-24-2018, 05:32 AM:
I always recommend filtered intake fans on the racks. Whereas I use QSC amps and they pull air from the rack and vent out the front, the racks never heat up. Furthermore, by pressurizing the rack, dust is avoided in the rack (or severely reduced) since it cannot settle and is instead prevented from coming into the rack via normal means.
It is actually pretty stark to see the difference between a rack that is pressurized versus one that isn't.